In a transmission device used in wireless communications, the amount of power consumption is large in a power amplifier (also referred to as PA) for amplifying an RF signal to transmit. Accordingly, in development of such a transmission device, improvement in power efficiency of the power amplifier is considered to be an important issue. In the recent communication standards, linear modulation with a stringent demand on signal distortion has become a mainstream, for the purpose of improving spectrum efficiency.
In a power amplifier, to maintain the linearity, the average output power is set such that the maximum instantaneous output power (hereafter referred to as the peak power) never exceeds the saturated output power. The ratio of the peak power to the average output power is called PAR (Peak-to-Average Ratio), which is an index of power amplifier control. Generally, in a power amplifier, the larger the PAR value of signals to be amplified is, the average output power needs to be set at a lower value with reference to the saturated output power (a backoff is to be set), in order to maintain the linearity.
However, in a general power amplifier, there is a problem in that, with decreasing the ratio of the average output power to the saturated output power, the power efficiency, which is the ratio of output power extracted from the power amplifier to supply power supplied to the power amplifier, decreases.
Each communication standard has its specific value for PAR of an RF signal. In high-speed wireless communications used in recent years, the PAR values are as large as about a few dB to a dozen dB (dB: decibel). Such high-speed wireless communications include CDMA (Code Division Multiple Access), WLAN (Wireless Local Area Network), terrestrial digital broadcasting, LTE (Long Term Evolution) and the like. In each of these high-speed wireless communications, the large PAR value determined in the communication standard is a cause of a large decrease in the power efficiency of a power amplifier.
To solve the problem of decrease in power efficiency of a power amplifier when the average output power is set at a low value, an outphasing technology has been actively studied in recent years.
In the outphasing technology, a transmission signal is separated into two constant-envelope signals (also referred to as outphasing signals), the two constant-envelope signals thus separated are amplified, and the two amplified constant-envelope signals are then combined together. According to the outphasing technology, a transmission signal is separated into two constant-envelope signals having no amplitude fluctuation and no necessity of backoff setting, and accordingly, the constant-envelope signals can be amplified using a nonlinear amplifier with high power efficiency. As a result, amplification of a transmission signal can be performed, achieving both preserving the output signal linearity and improving the power efficiency.
However, the outphasing technology has a problem of increase in amplitude error when the output power is low. Technologies for solving such a problem of an outphasing type PA are disclosed in Patent Literature 1 (PTL 1) and Patent Literature 2 (PTL 2).
PTL 1 discloses a method in which a phase adjuster is disposed at a preceding stage of a power amplifier and, accordingly, correction of an outphasing angle α is performed by means of a phase adjusting function of the phase adjuster.
PTL 2 discloses a method which performs mode switching such that outphasing operation of amplifying constant-amplitude RF signals by power amplifiers is performed when the output power is high, while a modulated RF signal with varying amplitude is directly amplified when the output power is low, instead of performing outphasing operation.
In another perspective, realizing a multiband scheme is one of important issues of wireless communication. As an example of multiband communication, Non-patent Literature 1 (NPL 1) discloses a Carrier Aggregation technology (hereafter referred to as a CA technology) which uses a plurality of fragmentary bands collected together. By thus combining the plurality of bands, the CA technology can preserve a broad bandwidth and also increase the transmission speed. For example, in an Inter-band Non-contiguous CA mode which uses carrier frequencies largely separated from each other, communication stability can be improved by performing simultaneous communications at a plurality of carrier frequencies having different propagation characteristics. If the CA technology can be employed, even when bands are allocated non-contiguously to a plurality of communication providers, or a plurality of communication providers share a band, it is possible to perform communications compatible with the respective cases.